Spectral domain-optical coherence tomography produces better results of retinal nerve fiber thickness imaging than scanning laser polarimetry, study shows

August 15, 2008

A recent study using a high-resolution spectral-domain optical coherence tomography (SD-OCT) to image and quantify retinal nerve fiber (RNF) loss in patients with glaucoma showed superior imaging results when compared with the imaging of a scanning laser polarimetry (SLP) device.

Key Points

Patients with glaucoma classically suffer from ganglion cell and RNF loss accompanied by glial cell loss, which results in the typical cupping of the optic disc. Because the structures are not retrievable, an early diagnosis is of paramount importance, according to Dr. Mardin. Typically, SLP and SD-OCT have been used to diagnose changes.

He and colleagues investigated and compared the imaging data taken of the retinal nerve fiber layer (RNFL) thickness with the SLP device and a new SD-OCT device (Spectralis HRA+OCT, Heidelberg Engineering) in 113 participants, including 33 healthy controls, 28 patients with ocular hypertension, 19 patients with pre-perimetric open-angle glaucoma, and 33 with perimetric glaucoma.

"The importance of the study is that for the first time, a high-resolution OCT has been used for the [RNFL] measurement," Dr. Mardin said. "This structure becomes thinner and thinner in glaucoma patients over time as the disease progresses, and a high-resolution device such as the [SD-OCT] proves to be very beneficial in precisely staging the extent of glaucoma disease, [which is] so important for the treatment and prognosis in patients."

The high-resolution SD-OCT instrument has been available on the market for a year now and, according to Dr. Mardin, it can provide superior imaging of the changes of RNFL compared with previously used devices. The SD-OCT device, which originally was constructed for the diagnostics of macular disease such as age-related macular degeneration, works very well to diagnose glaucoma in patients, he said. Dr. Mardin said that although the high-resolution instrument still is going through some growing pains in terms of the immature software that it uses, those small obstacles soon will be overcome.

"Two or 3 years ago, we still had a resolution of about 12 µm with the older OCT device, but now, with this new high-resolution OCT device, we can get a resolution of up to 7 µm-nearly double the resolution," Dr. Mardin said. "The images we are getting with this device show a very sharp delineation of the nerve fiber layer and, therefore, the structures can be measured much more precisely and accurately."

According to Dr. Mardin, the noise of the new device is much less when compared with that of the previously used technology, such as the SLP and the earlier version of the OCT. If the images have a low resolution, the result is a blurry image, which is a disadvantage in the accurate assessment of the level of RNFL degradation.

"We now can be much more precise with our measurements in our glaucoma patients. This is especially important in our . . . patients [whose disease is in the early stages], as early recognition of the disease is crucial," Dr. Mardin said.

Some ophthalmologists may argue that the OCT is not intrinsically useful for patients with glaucoma who present with an elevated IOP and symptoms such as papillary excavations and scotomas in the perimeter (considered late symptoms of the disease). In these typically standard glaucoma cases, one does not need the improved instrumentation, he said.

The new device, however, can be extremely helpful in cases of low-pressure glaucoma, that is, in patients who present with IOP in the standard deviation range of IOP and with no other symptoms such as papillary excavation or scotomas in the perimeter, Dr. Mardin said.

The previously used SLP technique still is considered the standard diagnostic tool in most private offices and mainly is used to diagnose the more tricky cases of glaucoma, such as low-pressure glaucoma, as well as to screen for patients whose conditions are borderline or for potential glaucoma cases.

Dr. Mardin said that the imaging of the RNFL and the optic disc, as well as the structures involved in glaucoma, should be done once a year because the changes that occur are very slow and very subtle in their course, occurring throughout decades.

According to Dr. Mardin, the new SD-OCT is an excellent device that will prove to be extremely useful in building research on the RNFL. The device also will be beneficial in diagnosing other eye diseases in which high-resolution imaging is important, he said. Whether it will be useful in everyday use still needs to be shown, but, according to Dr. Mardin, the SD-OCT is the best technology available on the market today.